Editing Jet propulsion (section)

==Physics==
Jet propulsion is produced by some [[reaction engine]]s or animals when thrust is generated by a fast moving [[jet (fluid)|jet]] of [[fluid]] in accordance with [[Newton's laws of motion]]. It is most effective when the [[Reynolds number]] is high—that is, the object being propelled is relatively large and passing through a low-viscosity medium.<ref name='Packard1972'>{{Cite journal| first1 = A.| title = Cephalopods and Fish: the Limits of Convergence| journal = Biological Reviews| volume = 47| issue = 2| last1 = Packard| pages = 241–307| year = 1972| doi = 10.1111/j.1469-185X.1972.tb00975.x| s2cid = 85088231}}</ref>

In animals, the most efficient jets are pulsed, rather than continuous,<ref name="ref_d">{{Cite journal | last1 = Sutherland | first1 = K. R. | last2 = Madin | first2 = L. P. | doi = 10.1242/jeb.041962 | title = Comparative jet wake structure and swimming performance of salps | journal = Journal of Experimental Biology | volume = 213 | issue = Pt 17 | pages = 2967–75 | year = 2010 | pmid =   20709925| url = https://authors.library.caltech.edu/19738/1/Sutherland2010p11195J_Exp_Biol.pdf | doi-access = free }}</ref> at least when the Reynolds number is greater than 6.<ref name="ref_e">{{Cite journal | last1 = Dabiri | first1 = J. O. | last2 = Gharib | first2 = M. | doi = 10.1098/rspb.2005.3109 | title = The role of optimal vortex formation in biological fluid transport | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 272 | issue = 1572 | pages = 1557–1560 | year = 2005 | pmid =  16048770| pmc = 1559837}}</ref>

===Specific impulse===
{{Main|Specific impulse}}
Specific impulse (usually abbreviated ''I''<sub>sp</sub>) is a measure of how effectively a [[rocket engine|rocket]] uses propellant or [[jet engine]] uses fuel. By definition, it is the [[impulse (physics)|total impulse]] (or change in [[momentum]]) delivered per unit of [[propellant]] consumed<ref name="QRG1">{{cite web|url=http://www.qrg.northwestern.edu/projects/vss/docs/propulsion/3-what-is-specific-impulse.html|title=What is specific impulse?|publisher=Qualitative Reasoning Group|access-date=22 December 2009|archive-date=4 July 2016|archive-url=https://web.archive.org/web/20160704233223/http://www.qrg.northwestern.edu/projects/vss/docs/Propulsion/3-what-is-specific-impulse.html|url-status=dead}}</ref> and is [[dimensional analysis|dimensionally equivalent]] to the generated [[thrust]] divided by the propellant [[mass flow rate]] or weight flow rate.<ref name="SINasa"/> If [[mass]] ([[kilogram]], [[pound-mass]], or [[slug (unit)|slug]]) is used as the unit of propellant, then specific impulse has units of [[velocity]]. If weight ([[newton (unit)|newton]] or [[pound-force]]) is used instead, then specific impulse has units of time (seconds).  Multiplying flow rate by the standard gravity ([[standard gravity|''g''<sub>0</sub>]]) converts specific impulse from the mass basis to the weight basis.<ref name="SINasa">{{cite web|url=http://www.grc.nasa.gov/WWW/K-12/airplane/specimp.html|title=Specific impulse|last=Benson|first=Tom|date=11 July 2008|publisher=[[NASA]]|access-date=22 December 2009|archive-url=https://web.archive.org/web/20100124223955/http://www.grc.nasa.gov/WWW/K-12/airplane/specimp.html|archive-date=24 January 2010|url-status=dead}}</ref>

A propulsion system with a higher specific impulse uses the mass of the propellant more effectively in creating forward thrust and, in the case of a rocket, less propellant needed for a given [[delta-v]], per the [[Tsiolkovsky rocket equation]].<ref name="QRG1" /><ref name="ars20130414">{{cite news|last=Hutchinson|first=Lee |title=New F-1B rocket engine upgrades Apollo-era design with 1.8M lbs of thrust |url=https://arstechnica.com/science/2013/04/new-f-1b-rocket-engine-upgrades-apollo-era-deisgn-with-1-8m-lbs-of-thrust/ |access-date=15 April 2013 |publisher=[[Ars Technica]] |date=14 April 2013 |quote=The measure of a rocket's fuel effectiveness is called its specific impulse (abbreviated as 'ISP'—or more properly Isp).... 'Mass specific impulse...describes the thrust-producing effectiveness of a chemical reaction and it is most easily thought of as the amount of thrust force produced by each pound (mass) of fuel and oxidizer propellant burned in a unit of time. It is kind of like a measure of miles per gallon (mpg) for rockets.'}}</ref> In rockets, this means the engine is more effective at gaining altitude, distance, and velocity. This effectiveness is less important in jet engines that employ wings and use outside air for combustion and carry payloads that are much heavier than the propellant.

Specific impulse includes the contribution to impulse provided by external air that has been used for combustion and is exhausted with the spent propellant. Jet engines use outside air, and therefore have a much higher specific impulse than rocket engines. The specific impulse in terms of propellant mass spent has units of distance per time, which is an artificial velocity called the "effective exhaust velocity". This is higher than the ''actual'' exhaust velocity because the mass of the combustion air is not being accounted for. Actual and effective exhaust velocity are the same in rocket engines not utilizing air.

Specific impulse is inversely proportional to [[Thrust specific fuel consumption|specific fuel consumption]] (SFC) by the relationship ''I''<sub>sp</sub> = 1/(''g<sub>o</sub>''·SFC) for SFC in kg/(N·s) and ''I''<sub>sp</sub> = 3600/SFC for SFC in lb/(lbf·hr).

===Thrust===

From the definition of specific impulse thrust in SI units is:

:<math>F = \dot m V_e</math>

where V{{sub|e}} is the effective exhaust velocity and <math>\dot m</math> is the propellant flow rate.